docs: def get_active_entry(user, select_for_update=False):
      """Returns the user's currently-active entry, or None."""
      entries = apps.get_model('entries', 'Entry').no_join
      if select_for_update:
          entries = entries.select_for_update()
      entries = entries.filter(user=user, end_time__isnull=True)
  
      if not entries.exists():
          return None
      if entries.count() > 1:
          raise ActiveEntryError('Only one active entry is allowed.')
      return entries[0]